Here is another round of the mempressure cgroup. This time I dared to
remove the RFC tag. :)

In this revision:

- Addressed most of Kirill Shutemov's comments. I didn't bother
implementing per-level lists, though. It would needlessly complicate the
logic, and the gain would be only visible with lots of watchers (which
we don't have for our use-cases). But it is always an option to add the
feature;

- I've split the pach into two: 'shrinker' and 'levels' parts. While the
full-fledged userland shrinker is an interesting idea, we don't have any
users ready for it, so I won't advocate for it too much.

And since at least Kirill has some concerns about it, I don't want the
shrinker to block the pressure levels.

So, these are now separate. At some point, I'd like to both of them
merged, but if anything, let's discuss them separately;

- Added documentation, describes APIs and the purpose;
- Implemented shrinker interface, this is based on Andrew's idea and
supersedes my "balance" level idea;
- The shrinker interface comes with a stress-test utility, that is what
Andrew was also asking for. A simple app that we can run and see if the
thing works as expected;
- Added reclaimer's target_mem_cgroup handling;
- As promised, added support for multiple listeners, and fixed some other
comments on the previous RFC.